Gregorian chant represents the continuing
musical tradition of the Catholic Church. In legend, at least, the
regularisation of Christian chant has been attributed to the sixth century Pope
St. Gregory the Great. Gregorian chant is, in fact, the form of plainchant that
largely but not entirely replaced local forms of chant during the Middle Ages.
Manuscript sources are preserved from the 10th and 11th centuries,
but these are clearly part of an earlier tradition. The term Gregorian chant is
generally acceptable, in popular usage, to describe the official chant of the
Church. This chant has musical value and interest in itself. Its historical
musical importance is immeasurable, since much of the liturgical music of the
Middle Ages and of the Renaissance was based on melodies drawn from this body
of music. In later years, particularly in the nineteenth century, the
connotations of elements of the chant continued as part of the common fund of
music to which composers might refer, notably in the chant for the Dies irae (Day of Wrath) from the Requiem
Mass, the opening notes of which provided a thematic allusion for Liszt's Totentanz and an idée fixe for
Rachmaninov.

Gregorian chant is monodic, modal and in free
rhythm. It has a single melodic line, without harmonic or polyphonic elements;
it came, at least, to make use of the eight church modes, scales represented by
the white notes of the modern keyboard and starting on D (Dorian mode), E
(Phrygian mode), F (Lydian mode) and G (Mixolydian mode), the names drawn from
the different ancient Greek modes; the rhythm of the chant follows that of the
words. It is possible to classify types of chant very simply as syllabic,
neumatic and melismatic. Syllabic chant takes one note to a syllable,
represented generally in the musical settings of the Psalms. Neumatic chant may
use groups of from two to four notes to a syllable, as often in the hymns of
Gregorian chant, and melismatic chant indicates the use of a large group of
notes for one syllable, as found in the florid music for the Alleluias of the
liturgy.

The liturgy of the Catholic church centres on
the Mass. The Ordinary of the Mass, the elements that remain constant
throughout the year, includes Kyrie(Lord
have mercy), Gloria (Glory be to
God in the highest), Credo (I
believe), Sanctus (Holy, holy,
holy) and Agnus Dei (Lamb of
God). The chants of the Proper of the Mass are those that differ from day to
day, according to the season or the saint or event to be celebrated. The Proper
consists of introit, gradual, alleluia, tract, offertory and communion, to
which may be added sequence and possible tropes, these last representing
additions to the liturgy, musical, verbal or both, many of which were removed
in the changes that took place as a result of the Council of Trent in the
sixteenth century.

The introit is to be sung, started by one or
more cantors according to the day, as the priest approaches the altar at Mass. Adorate Deum (Worship God, all you angels:
Sion has heard and is glad) is the introit for the Third Sunday after the
Epiphany. Da pacem (Grant peace,
O Lord, to those that worship you) is the introit for Mass of the Eighteenth
Sunday after Pentecost. Dominus illuminatio
mea (The Lord is my light and my salvation) is the introit for the
Fourth Sunday after Pentecost and Laetetur
cor (Let the heart of those that seek the Lord rejoice) is the
introit for Mass on the Friday after the Fourth Sunday in Quadragesima (the last Friday before
Passion Sunday).

The gradual and the alleluia come after the
chanting of the Epistle, bridging the gap between it and the chanting of the
Gospel. Dirigatur (Let my prayer
go up as incense) is the gradual for Mass on the Nineteenth Sunday after
Pentecost, elaborately melismatic in form. Domine,
Dominus noster (O Lord, Our Lord, how wonderful is your name) is the
gradual for the Ninth Sunday after Pentecost and lacta cogitatum tuum (Casty our thoughts on the Lord) is for
the Sunday within the Octave of the Feast of the Sacred Heart. Laetatus sum (I was glad when they said to
me) is the melismatic introit for Mass on the Fourth Sunday of Lent.

The highly melismatic alleluia verse Adorabo (I shall worship in
your holy temple) is part of the Mass for the Dedication of Church and the De profundis (Out of the depths) is from
the Mass for the Twenty- hird Sunday of Pentecost. Deus judex justus (God, just judge) follows the gradual lacta cogitatum tuum on the Sunday within
the Octave of the Sacred Heart and Laudate
Deum (Praise God) is the alleluia verse for the Second Sunday after
the Epiphany.

The offertory follows the singing of the
Credo, preceding the offering of bread and wine. De profundis (Out of the depths) is the offertory of the
Twenty-Third Sunday after Pentecost, its text slightly different from the
alleluia verse on the same day. Domine
convertere (Turn, O Lord) is the neumatic offertory for Mass on the
Sunday within the Octave of Corpus Christi, while Jubilate Deo (Rejoice in God) is the offertory of the Mass
of the Second Sunday after the Epiphany. Justitiae
Domini (The true justice of the Lord) is to be sung as the offertory
on the Third Sunday of Lent.

The communion verses are chanted at the
communion in the later part of the Mass Circuibo
(I shall go about and sacrifice) is the communion verse for Mass on the Seventh
Sunday after Pentecost and Dominus dicit:
Implete hydrias (The Lord says: fill the water-jars), a reference to
the Marriage Feast at Cana, the Gospel for the day, is for the Second Sunday
after the Epiphany. Dominus firmamentum meum
(The Lord is my strength) is the communion verse for the Fourth Sunday after
Pentecost and Qui manducat (He
who eats my flesh) for the Ninth Sunday. Gustate
etvidete(Taste and see how gracious is the Lord) is sung on the
Eighth Sunday of the same season.